btrfs: handle errors from select_reloc_root()

Currently select_reloc_root() doesn't return an error, but followup
patches will make it possible for it to return an error.  We do have
proper error recovery in do_relocation however, so handle the
possibility of select_reloc_root() having an error properly instead of
BUG_ON(!root).

I've also adjusted select_reloc_root() to return ERR_PTR(-ENOENT) if we
don't find a root, instead of NULL, to make the error case easier to
deal with.  I've replaced the BUG_ON(!root) with an ASSERT(0) for this
case as it indicates we messed up the backref walking code, but it could
also indicate corruption.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -2024,8 +2024,14 @@ struct btrfs_root *select_reloc_root(struct btrfs_trans_handle *trans,
 		if (!next || next->level <= node->level)
 			break;
 	}
-	if (!root)
-		return NULL;
+	if (!root) {
+		/*
+		 * This can happen if there's fs corruption or if there's a bug
+		 * in the backref lookup code.
+		 */
+		ASSERT(0);
+		return ERR_PTR(-ENOENT);
+	}
 
 	next = node;
 	/* setup backref node path for btrfs_reloc_cow_block */
@@ -2196,7 +2202,10 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 
 		upper = edge->node[UPPER];
 		root = select_reloc_root(trans, rc, upper, edges);
-		BUG_ON(!root);
+		if (IS_ERR(root)) {
+			ret = PTR_ERR(root);
+			goto next;
+		}
 
 		if (upper->eb && !upper->locked) {
 			if (!lowest) {